1. Field of the Invention
The present invention relates to a method for manufacturing a magnetic recording medium used for a hard disk device (HDD) or other devices and to a magnetic recording-reproducing apparatus.
2. Description of the Related Art
Recently, applicability of magnetic recording devices, such as magnetic disk devices, flexible disk devices and magnetic tape devices, has increased significantly and their importance has also increased. Recording density of magnetic recording media used for these devices has been increased significantly. With the advent of a magnetoresistive head and partial response maximum likelihood (PRML) technology, surface recording density has improved still more significantly. In recent years, recording heads, such as GMR heads and TMR heads, have also been introduced, which further increase the surface recording density by about twofold a year.
There is a demand to further increase recording density of these magnetic recording media. It is therefore necessary to increase coercive force, a signal-to-noise ratio (SNR) and resolution of magnetic layers. In recent years, efforts have been made to increase surface recording density by increasing linear recording density and track density.
The most recent magnetic recording media have track density of as high as 110 kTPI. As the track density increases, however, magnetic recording information between adjacent tracks begins interfering with each other, which may easily cause a problem that a magnetizing transition area of a border area becomes a noise source that decreases the SNR. The decrease in the SNR causes a decrease in a bit error rate, which is an obstacle to an improvement in recording density.
In order to increase surface recording density, it is necessary to provide reduced-sized recording bits on the magnetic recording medium, each recording bit having maximum possible saturation magnetization and maximum possible magnetic film thickness. There is a problem, however, that the reduced-sized recording bit has a small magnetizing minimum volume per 1 bit and recorded data may disappear due to flux reversal caused by heat fluctuation.
Since adjacent tracks are close to each other in a high track density configuration, a significantly precise track servo technique is necessary for a magnetic recording device. Usually, information is recorded on a larger number of tracks and reproduced in a smaller number of tracks in order to avoid influence from adjacent tracks as much as possible. In this manner, however, although influence between the tracks can be controlled to the minimum, it is difficult to obtain a sufficient reproduction output and thus to provide a sufficient SNR.
In order to avoid a heat fluctuation problem, provide a sufficient SNR and to provide sufficient output, an attempt has been made to form an uneven configuration along the tracks on the surface of the recording medium so as to physically separate the recording tracks from one another to increase the track density. Such a technique is usually called a discrete track process and a magnetic recording medium manufactured by that method is called a discrete track medium. An attempt has also been made to provide a “patterned medium” which has further divided data areas in a track.
An exemplary discrete track medium is a magnetic recording medium, which is formed on a non-magnetic substrate on which an uneven pattern is formed. On the magnetic recording medium, a physically-separated magnetic recording track and a servo signal pattern are formed (see, for example, Patent Document 1).
The disclosed magnetic recording medium includes a ferromagnetic layer formed on an uneven configuration of a substrate via a soft magnetic layer. A protective film is formed on the surface of the ferromagnetic layer. The magnetic recording medium has, in its projecting area, a magnetic recording area which is physically separated from the surrounding areas.
In the disclosed magnetic recording medium, since formation of a magnetic wall in the soft magnetic layer can be avoided and influence of the heat fluctuation can be prevented, there is no interference between adjacent signals. Thus, a high-density magnetic recording medium with less noise can be provided.
The discrete track process includes forming tracks after a magnetic recording medium consisting of several thin film layers is formed, or forming an uneven pattern directly on a substrate surface or on a thin film layer for forming tracks and then forming a thin magnetic recording medium film (see, for example, Patent Documents 2 and 3).
The former process, a magnetic layer process, has a following problem. Since physical processing is performed on a surface of a finished medium, the medium is easily contaminated during the manufacturing process and the manufacturing process becomes significantly complicated. The latter process, an embossing process, also has a following problem. Although the medium is not easily contaminated during the manufacturing process, since an uneven configuration formed on a substrate is taken over to a film formed thereon, levitation pose and levitation height of a head which records and reproduces on a medium become unstable.
As another approach, a method of forming an area between magnetic tracks of a discrete track medium by injecting nitrogen ions and oxygen ions into a previously formed magnetic layer or by irradiating with laser so as to change property of that area is disclosed (see, for example, Patent Documents 4 to 6).    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2004-164692    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2004-178793    [Patent Document 3] Japanese Unexamined Patent Application, First Publication No. 2004-178794    [Patent Document 4] Japanese Unexamined Patent Application, First Publication No. H5-205257    [Patent Document 5] Japanese Unexamined Patent Application, First Publication No. 2006-209952    [Patent Document 6] Japanese Unexamined Patent Application, First Publication No. 2006-309841
In manufacturing a discrete track medium or a patterned medium having a magnetically-separated magnetic recording pattern described above, the magnetic layer is patterned by the following methods (1) and (2).
(1) A patterned mask layer is formed on a surface of a magnetic layer and the magnetic layer is physically processed through ion milling or other means using the mask layer.
(2) A magnetic layer is partially doped with ions with a mask layer provided thereon and magnetic property of the magnetic layer is partially modified to form a magnetic recording pattern.
The method (1) has, however, a problem that since the mask layer itself is etched by the ions when the magnetic layer is subject to ion milling, an edge portion of the mask layer is formed in a forward tapered shape. An angle of a portion of the forward tapered shape with respect to the vertical direction becomes gradually large so that a cross-section of the magnetic layer to be processed may be formed in a forward tapered shape. In this case, since the edge portion of the magnetic layer is formed in a forward tapered shape, which may provide ill-defined boundaries in the magnetic recording pattern.
The method (2) also has a problem that, during ion doping into the magnetic layer, the mask layer is etched by the ions and an ill-defined pattern of the mask layer is gradually formed. Further, the doped ions are scattered in the magnetic layer, which may provide ill-defined boundaries in the magnetic recording pattern.